It is well recognized that most metal alloy compositions solidify dendritically. That is dendritic or tree-like particles grow from nuclei as the alloy composition is cooled below the liquidus temperature. It is also well known that certain advantages are provided by fragmenting dendritic particles or preventing dendritic growth during solidification to form non-dendritic or degenerate dendritic particles having a generally spheroidal or ellipsoidal shape. More specifically, it has been discovered that various processing and physical property advantages can be achieved by casting or otherwise forming metal components from a non-dendritic, semi-solid metal slurry. The non-dendritic metal particles in the semi-solid slurry provide substantially reduced viscosity for a given solids fraction as compared with a semi-solid metal alloy composition containing dendritic particles. Often the difference in viscosity is several orders of magnitude.
The resulting benefits of non-dendritic semi-solid metal forming include higher speed part forming, high speed continuous casting, lower mold erosion, lower energy consumption, improved mold filling, reduced oxides that provide improved machinability in the finished metal components, and less gas entrapment resulting in reduced porosity. Other advantages of casting or otherwise forming metal components from a semi-solid slurry include less shrinkage during forming of the metal components, fewer voids and lower porosity in the formed metal components, less macrosegregation, and more uniform mechanical (e.g., strength) properties. It is also possible to form more intricate parts using non-dendritic, semi-solid alloy compositions during casting or other forming techniques. For example, parts having thinner walls with improved strength properties are possible.
Non-dendritic, semi-solid slurries for industrial casting and other metal forming processes have been prepared using mechanical mixing during cooling of a liquid metal alloy composition below the liquidus temperature of the alloy composition. Other techniques that have been utilized include electromagnetic stirring during cooling (typically for continuous casting processes), cooling a liquid metal composition while passing it through a torturous channel, long thermal treatments in the semi-solid temperature region, and others. These techniques are well known and have been advantageously employed in various commercially important applications.
More recently non-dendritic, semi-solid slurries have been created by relying upon the pouring of low superheated molten alloy into relatively cool containers (e.g. a crucible or the cold chamber of a die casting machine). These processes rely upon the cooling of the alloy composition from above the liquidus temperature to below the liquidus temperature during the pouring action as the alloy contacts the vessel walls. The process is effective in creating non-dendritic semi-solid slurries; however, there are process limitations. First, the process relies upon heat extraction from the vessel walls. It is difficult to control heat removal using this technique because of the changing temperature of the walls and the discrete surface area of the cylinder. Second, convection is created by the pour; therefore, if the alloy is introduced at too high of a temperature, convection forces dissipate before the alloy cools through the liquidus, preventing the formation of non-dendritic slurries.
Commercial products have included various aluminum and magnesium alloy components for automotive applications, such as master brake cylinders, and various components for steering and suspension systems. Other actual or potential applications include rocker arms, engine pistons, wheels, transmission components, fuel system components, and air conditioner components.
A problem with known techniques of forming a non-dendritic semi-solid metal slurry using mechanical agitation is that the surfaces of the agitator are wetted by the liquid metal in the slurry. As a result, some of the liquid metal from the slurry sticks to the surfaces of the agitator when it is removed from the slurry. Any liquid metal that wets or sticks to the surfaces of the agitator and/or the vessel quickly solidifies and forms a metal coating that must be removed before the agitator and/or vessel may be reused for preparation of more non-dendritc, semi-solid metal slurry. Removal of metal deposits from the surfaces of the agitator is typically difficult, time consuming, expensive, and leads to lower production rates. Materials having a reduced wettability are typically unsuitable for use in handling liquid metal alloy compositions (e.g., because they lack adequate mechanical properties at the high temperatures associated with the production of non-dendritic, semi-solid metal slurries) and/or do not have a sufficiently high thermal conductivity suitable for rapidly withdrawing heat from the non-dendritic, semi-solid metal slurries. Reduced wettability has been achieved by applying low wettability coatings to the surfaces of metal agitators. Boron nitride coatings have been used on agitator and/or vessel surfaces to successfully reduce wettability without adversely reducing thermal conductivity. However, the boron-nitride coatings lack structural strength, and require periodic replacement.
Another problem with conventional processes for preparing non-dendritic, semi-solid metal alloy compositions having a relatively high solids content (e.g., greater than about 10%) is that a considerable amount of time is typically required to cool the slurry to the desired solids content. Typically, agitation of the alloy composition occurs in a ceramic vessel or a preheated vessel in order to prevent nucleation and solid formation at the walls of the container or vessel in which the agitation is performed. As a result, cooling occurs relatively slowly, resulting in lengthy process times and reduced production. Rapid cooling can be achieved using a cool vessel having adequate mass, thermal conductivity and heat capacity. However, this can lead to unacceptably high temperature gradients that are not conducive to formation of non-dendritic semi-solid slurries, and/or cooling of the alloy composition to a temperature that is unsuitable for forming the alloy composition into a desired component.
U.S. Pat. No. 6,645,323 discloses a skinless metal alloy composition that is free of entrapped gas and comprises primary solid discrete degenerate dendrites homogenously dispersed within a secondary phase. The disclosed alloy is formed by a process in which metal alloy is heated in a vessel to render it a liquid. Thereafter, the liquid is rapidly cooled while being vigorously agitated under conditions that avoid entrapment of gas while forming solid nuclei homogenously distributed in the liquid. Cooling and agitation are achieved utilizing a cool rotating probe that extends into the liquid. Agitation is ceased when the liquid contains a small fraction solid or the liquid-solid alloy is removed from the source of agitation while cooling is continued to form the primary solid discrete degenerate dendrite in a liquid secondary phase. The solid-liquid mixture is then formed such as by casting. A problem with the process disclosed in U.S. Pat. No. 6,645,323 is that the cool rotating probes utilized for cooling and agitation tend to become coated with liquid metal that sticks to the surfaces of the agitator. As a result, the agitator as described in this patent requires frequent cleaning and/or replacement. Further, there remains a need for improving control over the amount of heat that is extracted from the aluminum alloy composition. In certain aspects of this invention, processes and apparatuses are provided for overcoming these deficiencies.